This paper presents both experimental and numerical results demonstrating the dramatic effect of a particular geometric discontinuity on the flow within a highly-loaded single-stage transonic compressor. This compressor features cantilevered variable-stagger stator vanes (VSV) with buttons at the outer radius partially standing out in the annulus, by a value of about 0.3 mm which amounts to 0.5% of vane height.
The first tests of this compressor revealed a very large corner flow separation at the stator outer radius. This feature could not be reproduced by steady 3D Navier-Stokes computations of the stator flow field with a smooth flowpath. However, using a simple numerical approach to account for the actual geometric discontinuity of the annulus, the computation then showed flow trends similar to that of the experiment. Also, a detailed analysis of the computed flow field indicated that the flow turning (both radial and tangential) due to the button blockage contributed toward strengthening the effect of secondary flows. Subsequently, after the button geometry was corrected to match the flowpath, further tests confirmed a much more satisfactory flow behaviour within the stator vanes leading to a one point improvement in compressor isentropic efficiency.
This study leads to the conclusion that ensuring a smooth flowpath can be critically important for the high stage leadings characteristic of advanced military applications. Moreover, the predictive value of 3D Navier-Stokes computations for the simulation of technological effects, such as VSV buttons / flowpath mismatch, is clearly demonstrated.